Journal of the Institute of Engineering, 2015, 11(1): 116-119 116 TUTA/IOE/PCU © TUTA/IOE/PCU Printed in Nepal Product of Ap Weight Functions Vinod Parajuli, Santosh Ghimire Department of Engineering Science and Humanities, Pulchowk Campus, Institute of Engineering Tribhuvan University, Nepal Corresponding author: [email protected] Abstract: In this paper, we first define Ap weight functions and then show that finite product of weight functions each raised with some power whose sum is one is also an Ap weight function. Keywords: weight function, Holder's inequality, Maximal function. 1. Introduction In 1970, Muckenhoupt characterized positive functions w for which the Hardy-Littlewood maximal operator M maps Lp(Rn, w(x)dx) to itself. Muckenhoupt's characterization actually gave the better understanding of theory of weighted inequalities which then led to the introduction of Ap class and consequently the development of weighted inequalities. Weighted inequalities are used widely in harmonic analysis. For more about the theory of weights and applications in harmonic analysis, refer [1, 4]. In order to prove the result, some definitions and results are in order: n Definition: A locally integrable function on R that takes values in the interval (0, ∞) almost everywhere is called a weight. So by definition a weight function can be zero or infinity only on a set whose Lebesgue measure is zero. We use the notation to denote the w-measure of the set E and we reserve the notation Lp(Rn,w) or Lp(w) for the weighted L p spaces. We note that w(E)< ∞ for all sets E contained in some ball since the weights are locally integrable functions. Definition: A function w(x)≥0 is called an A1 weight if there is a constant C1>0 such that M(w)(x)≤ C1 w(x) where M(w) is uncentered Hardy-Littlewood Maximal function given by 1 sup . If w is an A1 weight, then the quantity (which∈ is finite)|| given by Parajuli and Ghimire 117 is called the A1 characteristic constant of w. Definition: Let 1 < p < ∞. A weight w is said to be of class Ap if is finite where is defined as n We remark that in the above definition of A1 and Ap one can also use set of all balls in R instead n of all cubes in R . Readers are suggested to read [4] for motivation, properties of Ap weights and much more about the Ap weights. Also refer [2] and [3] for more properties on A1 and Ap weight function. 2. Holder's inequality Let p and q be two real numbers such that p>1 and and if and Then and . Now we state our main result. Suppose that weight with for some and let be such that We then show that the product function given by is an Ap weight function where p is the maximum value of The proof will be done in following steps: (i) We prove that for all j. (ii) We show that the following inequality holds: (iii) We will use (ii) and the Holder's inequality to show 118 Product of Ap Weight Functions (iv) Finally by (i) and (iii) we prove that . Since for all j, using the decreasing nature of , we have for all j. This proves (i). To prove (ii) we do as follows. If for some j then the equality holds. Assuming for all j and letting one gets We note that we used the concavity of in the above expression. Since is an increasing function, it follows that . This implies From the above inequality (ii) follows. Finally we prove (iii). Let By (ii) we have, We write = Let and Applying the Holder's inequality, we obtain Parajuli and Ghimire 119 ∑ ∑ ∏ || ∏ Continuing in this manner, one get where C is a constant. Therefore, by (i), 1 1 Taking supremum over the cube|| Q in the above inequa||lity, we get ∈. References [1] Ba~nelos R and Moore CN (1991), Probabilistic Behavior of Harmonic Functions, Birkhauser Verlag. [2] Ghimire S (2014), Weighted Inequality, Journal of Institute of Engineering, 10(1), 121-124. [3] Ghimire S (2014), Two Different Ways to Show a Function is an A1 Weight Function, The Nepali Mathematical Sciences Report, 33(1 & 2), 2014. [4] Grafakos L (2009), Modern Fourier Analysis, Second Edition, Springer. .